#include <stdio.h>
#include <math.h>
#define p (a+b+c)/2

int main(void) {
	float a, b, c;
	scanf("%f %f %f", &a, &b, &c);
	printf("%.1lf", sqrt(p * (p - a) * (p - b) * (p - c)));
	return 0;
}